Acute Pain and Opioids: Across the ages

From April 9 – 12 2006, international and local pain experts gathered in Melbourne for the 26th Annual Scientific Meeting of the Australian Pain Society. The theme was “Pain Across the Lifespan.” Dr Pam MacIntyre from the Acute Pain Service, Adelaide Hospital gave an enlightening speech on “Acute Pain and Opioids: Across the Ages”.

From over 3,000 years ago to the present day, opioids have been the mainstay of acute pain management. However, even basic forms of bench work did not start until the 17th century. For example, in 1665, Sir Christopher Wren became the first person to describe the intravenous injection of opium using a goose quill and pig’s bladder. He noted that the animal became very drowsy, but survived the experiment. Despite these studies, administration of opioids by injection did not become commonplace until the introduction of the hypodermic needle and syringe in 1853. During the 1950s and 1960s, research began to focus on the site of action of opioids, but it is really since the 1970s, following identification of opioid receptors in the CNS and existence of endogenous opioids, that we began to have a better insight into how these drugs worked.From that time there have been significant and rapid advances in the understanding of opioids and in the ways in which they could be used in order to better manage acute pain. We also have newer and more effective methods of opioid delivery, such as PCA and epidural analgesia.In addition, we have gained a much better understanding of the differences in opioid requirements across the lifespan from neonate to the elderly patient. In rats, ageing is associated with reductions in opioid receptor density and increases in receptor affinity. In the human adult, the dose of opioid necessary for effective treatment of acute pain decreases as patient age increases. The decrease seems to have a significant pharmacodynamic component, as it is much greater than can be accounted for by age-related changes in physiology. At the other end of the lifespan, age also appears to be the most important factor determining opioid requirements. Clinically, neonates and infants seem to be more sensitive to morphine given for postoperative pain. This probably results from a number of factors including alterations in opioid pharmacokinetics, developmental changes in the opioid recetor (including distribution and function) and changes in the way the developing nervous system processes pain.Still newer ways of delivering opioids are emerging. Developed at the bench and still to be used widely at the bedside, these include iontophoretic fentanyl PCA and extended-release epidural morphine. Iontophorectic fentanyl PCA has the advantage of not requiring a PCA pump, but there is also no ability to alter the dose delivered (a very useful feature of currently used PCA devices). Extended-release epidural morphine means that a single dose can result in up to 48 hours of pain relief, but the dose is not titratable and coadministration of local anaesthetics is currently not advised (studies suggest that if patient outcome is to be improved following epidural analgesia, local anaesthetics should at least be a component of the infusion solution).Will these newer systems become common clinical practice? Are the bench to bedside advances now being made in opioid analgesia as clinically significant now as the advances that were made in the last 30-40 years?Please click here to read the full lecture series.